The present invention addresses the need for new austenitic steel compositions with higher creep strength and higher upper temperatures as compared to the presently used H-Series of cast stainless steels. Heat-resistant cast austenitic stainless steels and alloys are the backbone of the chemical, petrochemical, heat-treating and metals processing industries today, with applications continuing to drive performance, durability and use-temperatures higher, while economics tries to lower the cost of such alloys. There is a significant and continued need for low-cost austenitic stainless steel alloys that can be used in the as-cast condition at high temperatures up to 1200° C. Alloys currently used for this purpose have a significant Nickel (Ni) content added (˜35-45 wt. %), a large Cobalt (Co) content (up to 15 wt. %), or a large Tungsten (W) or Molybdenum (Mo) content. For these alloys, creep properties at 1200° C. can vary widely within the composition ranges specified in these inventions and a better definition of alloy compositions is needed for optimum creep properties for the temperature range of operation.
The alloy compositions of the present invention are designed specifically for improved creep properties at high temperatures of up to and including about 1200° C. Microstructure is a unique characteristic of the alloys of the present invention and forms the basis of their high temperature strength. The key problems solved by the present invention are the reduction of Cobalt (Co) content and the need for only small quantities of other alloying elements. The microstructure design creates a stable austenite resistant and an optimum combination of MC and M23C6 carbides, which is promoted through the addition of alloying elements.
For service temperatures above 850° C. to 900° C., the dominant alloy for use in steam methane reforming or ethylene cracking applications was initially HK-40 stainless steel. More recently modified or micro-alloyed HP stainless alloys have been used for these applications. In 1941, the Alloy Casting Institute introduced the classifications used today, designating the heat-resistant grades as H-grades and the corrosion-resistant stainless steels and alloys as C-grades. Fairly complete descriptions of properties, compositions, and standard industrial practice for the various grades of cast austenitic stainless steels and alloys can be found in handbooks or data available from the Steel Founders Society of America, ASM International, The Nickel Development Institute, The Specialty Steel Industry of North America, and/or data sheets compiled by the various leading alloy casters. The HK-40 stainless steel is essentially a Fe-25Cr-20Ni-0.4C alloy, whereas HP-40 stainless alloy is Fe-25Cr-35-Ni-0.4C, with more creep-resistant modifications occurring in the HP modified (+Nb) or the HP micro-alloyed (+Nb+Ti or +Nb+Zr) materials. In the 1960s and 1970s, efforts to improve the carburization-resistance of the HK-40 steels led to additions of up to 2% Silicon (Si) and increases in Nickel (Ni) (IN-519, 25Cr-25Ni-1.5Nb and HP alloys), while efforts to increase the strength and creep-resistance added Niobium (Nb). Costly upgrades of the modified HP alloys include additions of Tungsten (W) and Cobalt (Co) to further increase the high-temperature strength.
The native microstructure established in these fully-austenitic alloys consists of dendritic structures of austenite matrix with finer dispersions of carbides (Cr-rich M23C6 or Nb-rich MC, depending on the alloy), and heavier clusters of NbC in the interdendritic regions (the last liquid to solidify) and dispersions of M23C6 along the seams between colonies of dendrites. Aging effects can vary, with little deleterious effects above 950° C. to 1000° C., particularly in the modified HP alloys, but with potential embrittlement (severe ductility loss at ambient temperatures) due to M23C6 films and/or sigma phase formation during prolonged exposure below 900° C., mainly in the HK-40 type alloy. Additions of Cobalt (Co) are made mainly to strengthen and stabilize the austenite matrix phase, while additions of Tungsten (W) promote solid solution strengthening as well as Tungsten Carbide (WC) formation. Ethylene cracking and radiant furnace tubes generally involve prolonged exposure at relatively steady temperatures, where creep-resistance and oxidation/corrosion resistance are the life-defining properties. However, materials processing applications of such alloys, including steel mill furnace rollers and retorts for calcining, involve more than just creep-strength, and must include thermal fatigue resistance to prevent surface cracking (critical for coiling drums) or catastrophic through-section fracture (retorts). While one can possibly use the more expensive chemical/petrochemical premium alloys for such materials processing equipment applications, the materials processing industries probably would be better served by improving the strength and aging resistance of the standard HK-40 grade steel to provide a more cost-effective solution.
Alloy development of complex engineering alloys based on single or multiple alloying element additions or changes over wide ranges can often be very labor intensive, time consuming and costly. Usually such traditional brute-force efforts produce only modest incremental improvements, and then such improvements must be further verified by testing relevant to real-time component service. Therefore, most applications engineers attempt to redesign components or to solve their materials problems by selecting alternate materials, and they only turn to traditional alloy development as a last resort.
A far more scientific and yet practical method of precise microstructural analysis and identification of the degradation/failure mechanisms was devised at Oak Ridge National Laboratory to improve the creep-resistance of 300-series austenitic stainless steels at about 700° C. to about 800° C. This method provided a framework for translating various single or combined alloying element effects directly into their effects on precipitation behavior or stability of the parent matrix phase (austenite). When this scientific knowledge of how to stabilize desirable phases and reduce or eliminate undesirable phases was coupled with a thorough knowledge of microstructure/properties relationships and failure mechanisms, precise microstructures were designed that produced outstanding long-term creep-resistance the first time those modified stainless steels and alloys were made. The alloying addition effects were classified as (a) direct reactant effects (i.e., Nb+C=NbC), (b) catalytic effects (i.e., solutes that enhance the formation of a phase even though they are not direct reactants, like Silicon (Si) enhancing the formation of Fe2Mo Laves phase or Boron (B) enhancing the formation of TiC or M23C6 carbides), (c) inhibitor effects (i.e., solutes that retard or prevent the formation of particular precipitate phases, like Carbon (C), Boron (B), and Phosphorus (P) retarding or preventing the formation of intermetallic phases like sigma, chi or Laves), and (d) interference effects (i.e., two or more phases competing for the same element to decouple or simplify phase behavior and control in complex alloys). Some of the microstructural effects that have been controlled to create extremely long-term creep-rupture resistance include: (a) the elimination of creep voids; (b) the promotion of fine dispersions of MC carbides; (c) the prevention of dissolution or coarsening of fine MC carbides; (d) the delay or prevention of the formation of embrittling grain-boundary intermetallic phases; and (e) the prevention of dislocation recovery or recrystallization (mainly for wrought alloys).
As discussed more fully below, the related art recognizes and discusses previous efforts to obtain the improved alloys of the present invention. However, these efforts suffered from various shortcomings, including the need for costly element additions and low creep lives at high temperatures.
For example, U.S. Pat. No. 3,627,516 describes Fe—Ni—Cr alloys with a preferred composition of 26% Chrominium, 32% Nickel, 0.4% Carbon, 1.1% Manganese, 1.2% Silicon, 0.08% Nickel, 1.2% Niobium and the rest Iron for use at temperatures ranging from 800° C. to 1200° C. This patent also notes that 0.5% to 1.5% Molybdenum was beneficial in some cases. The present invention, on the other hand, identifies alloys with higher Nickel content. Niobium (Nb) contents in the present invention are also higher than in the alloys identified in U.S. Pat. No. 3,627,516 (thus lower overall costs). Molybdenum (Mo) contents are also lower in the present invention, which, again, is different from the related art.
Similarly, U.S. Pat. No. 3,758,294 discloses improved alloys with additions of 0.18% Nitrogen, 1.7% Tungsten, and 1.3% Niobium. The preferred embodiment including resistance to carburization included 0.4 wt. % Carbon, 25 to 28 wt. % Chromium, 32 to 36% Nickel, 0.5 to 1.0% Manganese, 1.2 to 1.6% Silicon, 1.4 to 2.0% Tungsten, 1.0 to 1.8% Niobium and 0.15% Nitrogen. Comparison of the creep life of the alloys identified in U.S. Pat. No. 3,627,516 and U.S. Pat. No. 3,785,294 with those in the present invention reveals that the present invention's new alloys possess improved creep properties at about 1200° C. Thus, improved properties were obtained with much lower alloying element additions, particularly Tungsten (W) and Niobium (Nb), and without the addition of Nitrogen (N).
U.S. Pat. No. 4,853,185 and U.S. Pat. No. 4,981,674 identify Fe—Ni—Cr alloys with 25-45% Nickel, 12-32% Chromium, 0.1 to 2.0% Niobium (minimum of nine times Carbon content), 2.0% Titanium max, 3% Silicon max, 0.05-0.5% Nitrogen, 0.02 to 0.2% Carbon, 2.0% Manganese max, 1.0% Aluminum max, 5.0% Tungsten/Molybdenum max, 0.02% Boron max, 0.2% Zirconium max, 5% Cobalt max, Yttruim, Lanthanum, Copper, REM up to 0.1% max. At least one Niobium, Tantalum, or Vanadium has to be present in the alloy along with Carbon and Nitrogen. Niobium (Nb) has to be added to a level of at least nine times the Carbon content. In contrast, the alloys of the present invention need no additions of Tantalum (Ta) or Vanadium (V) with very small simultaneous additions of Niobium (Nb), Tungsten (W), and Molybdenum (Mo). In addition, no Nitrogen (N) is intentionally added in the alloys of the present invention and thus the concentration can be much lower than 0.05%. Furthermore, the Carbon (C) levels in the present invention's alloys are much larger than those identified by Rothman et al.
In related art, U.S. Pat. No. 4,615,658 teaches a material for gas turbine shrouds that contains 0.35 to 0.5 wt. % Carbon, 22 to 24 wt. % Chromium, 24 to 26 wt. % Nickel, 0.15 to 0.35 wt. % Titanium, 0.2 to 0.5 wt. % Niobium, 0.1 to 1.2 wt. % Manganese, less than 0.8 wt. % Silicon and the balance Iron. Also suggested were additions of 0.05 to 5 wt. % rare-earth elements, 5 to 20 wt. % Cobalt, less than 7 wt. % Tungsten, and less than 7 wt. % Molybdenum. The present invention does not require Cobalt (Co) and rare-earth elements. Further, U.S. Pat. No. 4,615,658 does not show the creep properties of the alloys at high temperatures, and the amount of Tungsten (W) required by the invention of U.S. Pat. No. 4,615,658 is significant, which influences the cost and other alloy properties.
U.S. Pat. No. 6,485,674 discloses a heat resistant austenitic stainless steel with 0.04 to 0.1% Carbon, not more than 0.4% Silicon no more than 0.6% Manganese, 20 to 27% Chromium, 22.5 to 32% Nickel, not more than 0.5% Molybdenum, 0.2 to 0.6% Niobium, 0.4 to 4.0% Tungsten, 0.1 to 0.3% Nitrogen, 0.002 to 0.008% Boron, less than 0.05% Aluminum, and at least one of Calcium or Manganese. The alloys of the present invention contain a larger amount of Carbon (C), are able to accept more Silicon (Si) and Manganese (Mn) do not need any Nitrogen (N) or Calcium (Ca) additions, and achieve excellent heat resistance at temperatures up to 1200° C.
U.S. Pat. No. 6,685,881 discloses an austenitic stainless steel for use up to 950° C. with good heat resistance and machinability. The compositions of those alloys are in the range of 0.2 to 0.4% Carbon, 0.5 to 2.0% Silicon, 0.5 to 2.0% Manganese, 8 to 42% Nickel, 15 to 28% Chromium, 0.5 to 7.0% Tungsten, 0.5 to 2.0% Niobium, up to 0.05% Titanium, up to 0.15% Nitrogen, 0.001 to 0.5% Selenium, up to 0.1% Phosphorus, and 0.04 to 0.2% Sulfur. The present invention alloys have good heat resistance up to about 1200° C. In addition, the Tungsten (W) and Niobium (Nb) contents of the present invention alloys are much lower with no intentional additions of Nitrogen (N). The creep lives of the alloys in the U.S. Pat. No. 6,685,881 are not specified.
In summary, the present invention increases the high-temperature strength and upper use temperature of H-Series of cast austenitic stainless steels without the requirement of certain costly element additions. The present invention may be used in, among other things, steam methane reformer tubes, ethylene cracking furnace tubes, radiant furnace tubes, steel mill furnace transfer rollers, and retorts for calcining. The result is a significant energy and cost savings each year.